Implantable medical devices are known in many forms and for treating many medical conditions. Examples include stents, grafts, filters, occluders, valve replacement prostheses and so on. Such devices are generally introduced into the patient endoluminally through a remote percutaneous entry point. In order to achieve this, the medical device is loaded onto a carrier at a distal end of an introducer assembly and held in a radially compressed configuration. The introducer assembly is fed into the patient's vasculature from the percutaneous entry point until its distal end is located at the treatment site. Once so positioned, the medical device is released from the carrier and expanded until the device engages the vessel wall to be held thereby. The device can be of a type which expands automatically, achieved by use of spring material, shape memory material and so on. Other types of device are plastically deformable and expanded by a separate mechanism, for instance by expansion of a delivery balloon on which the device is held in crimped form.
It may be important that the medical device applies, in use, a constant force against the walls of the vessel in which it is located. This ensures good patency to the vessel wall, that is, a good seal between the device and the wall tissue, in order to stop leakage around the device. The application of constant force also may ensure that the device does not migrate or rotate out of alignment over time.
The force produced by the above-mentioned medical devices may be a mechanical force, be it by spring force of the components of the device or by relative mechanical stiffness in the case of a plastically deformable device. This may require the devices to have a certain structural strength and as a result a certain volume of material, resulting in increased device profile and reduced compressibility for delivery purposes. Furthermore, the structure of such devices can impart unnatural forces on the vessel wall, the most common being a vessel straightening force acting against the natural curvature of the vessel and/or excessive expansion pressure on the vessel. Such forces can lead to restenosis of the vessel.
There is also a growing desire to have implantable medical devices which are biodegradable. Polymers and similar materials generally have better biodegradability than metals and metal alloys. However, polymers tend to have worse expansion properties, being generally unable to produce equivalent opening forces relative to their metal and metal alloy counterparts, and can also suffer from loss of springiness when kept compressed for any length of time, for instance between loading onto an introducer assembly and eventual deployment into a patient.
Some examples of implantable medical devices are disclosed in US 2011/0257724, US 2006/0212113, U.S. Pat. No. 8,449,604 and U.S. Pat. No. 7,722,668.